1. Field of the Invention
The present invention relates to an image sensor having a photo diode and a method for manufacturing the same, and more particularly, to an image sensor having a photo diode for improving sensibility, junction leakage, and electron capacity, and a method for manufacturing the image sensor.
2. Description of the Related Art
A pinned photo diode is used for a complementary metal-oxide semiconductor (CMOS) image sensor, which is manufactured by CMOS processes, or a charge coupled device (CCD) image sensor to detect light for generating and accumulating photo electrodes. Since the pinned photo diode is formed in a PNP or NPN junction structure buried in a substrate, the pinned photo diode is referred to as a buried photo diode. The CMOS image sensor is subject to less power consumption than the CCD image sensor and is manufactured by a simpler process. Moreover, the CMOS image sensor can be formed together with a signal processing circuit in one chip, making it attractive as a next-generation image sensor.
The CMOS image sensor having the above-described pinned photo diode will be briefly described with reference to FIGS. 1 and 2.
FIG. 1 is a circuit diagram of a unit pixel Pix in a conventional image sensor, made up of one photo diode PD and four MOS transistors. The source (or drain) of a transfer transistor Tx is connected to the photo diode PD, and the source of a reset transistor Rx is connected to the drain (or source) of the transfer transistor Tx. A floating-diffusion capacitor Cfd is formed between the drain (or source) of the transfer transistor Tx and the source of the reset transistor Rx. The gate of a drive transistor MD is connected to the source of the reset transistor Rx and the drain (or source) of a select gate Sx is connected to the source of the drive transistor MD. In this case, a source voltage VDD is supplied to the drains of the reset transistor Rx and the drive transistor MD. A load transistor Vb is connected to the source (or drain) of the select gate Sx outside the unit pixel Pix, and the source (or drain) of the select gate Sx operates as the output of the image sensor.
FIG. 2 illustrates a semiconductor substrate in which the unit pixel of the above-described image sensor is integrated. In FIG. 2, only the photo diode, the transfer transistor, and the reset transistor are illustrated.
As shown in FIG. 2, an isolation layer 11 is formed on the substrate 10 by a conventional method. After a gate oxide layer 12 and a conductive layer 14 are deposited on the semiconductor substrate 10, portions of the layers 12 and 14 are patterned to form a transfer gate Tg and a reset gate Rg.
P-type impurities, e.g., boron ions, are implanted into the drain region (or the source region) of the transfer transistor, which is at one side of the transfer transistor, forming a p-type photo diode region 15. Then, n-type impurities, e.g., group V impurity ions such as arsenic or phosphorus ions, are implanted into a lower portion of the p-type photo diode region 15, forming an n-type photo diode region 20. As a result, p-type and n-type photo diodes are completed. Next, spacers 22 are formed on both walls of the transfer gate Tg and the reset gate Rg by a conventional blanket etching. By implanting the n-type impurities into both sides of the spacers 22, a common source region 24 and a drain region 26 of the reset transistor are formed.
However, forming the conventional n-type photo diode region 20 by implanting a single type of impurities, such as the arsenic or phosphorus ions, causes the following problems.
When the n-type photo diode region 20 is formed by implanting arsenic ions, a depletion distance becomes smaller because the projection distance xcex94Rp and diffusivity of the arsenic ions are small. Consequently, a high concentration of arsenic ions is maintained in each unit area, increasing junction capacitance. As a result, the electron capacity of the photo diode is improved. However, since the projection distance xcex94Rp and diffusivity of the arsenic ions are small, it is difficult to form the n-type photo diode region 20 over a large area. Accordingly, the sensitivity of the photo diode is reduced, and it is likely that junction leakage occurs with an abrupt junction profile.
When the n-type photo diode 20 is formed by implanting phosphorus ions, the photo diode is easily formed over a large area, due to a large projection distance xcex94Rp and diffusivity, improving sensitivity. In addition, the junction leakage is reduced by smoothing the profile on a junction interface. However, since the projection distance xcex94Rp and diffusivity of the phosphorus ions are large, the depletion distance is increased, thereby reducing the junction capacitance and the concentration of phosphorus ions in each unit area. As a result, the electron capacity of the photo diode is reduced.
To solve the above-described problems, it is an objective of the present invention to provide an image sensor having a photo diode with improved sensitivity, junction leakage and electron capacity.
It is another objective of the present invention to provide a method for manufacturing the image sensor having the photo diode.
In one aspect, the present invention is directed to an image sensor having a photo diode. The image sensor includes a semiconductor substrate and a p-type photo diode region formed on a selected region of the semiconductor substrate. A first n-type photo diode region is formed underneath the p-type photo diode region, contacting an interface of the p-type photo diode region, and a second n-type photo diode region is formed to surround the first n-type photo diode region. In accordance with the invention, impurities of the first n-type photo diode region have smaller projection distance and diffusivity than impurities of the second n-type photo diode region.
The impurities of the first n-type photo diode region may be arsenic ions, the impurities of the second n-type photo diode region may be phosphorus ions, and the impurities of the p-type photo diode region may be boron ions.
It is preferable that the second n-type photo diode region be formed over a larger area than the first n-type photo diode region.
An image sensor having a semiconductor substrate according to a second embodiment of the present invention comprises a transfer gate and reset gate formed on predetermined portions of the semiconductor substrate with a specific distance therebetween. A photo diode region is formed on one side of the transfer gate, a common source region is formed between the transfer gate and the reset gate, and a drain region of the reset transistor is formed on the other side of the reset gate. In accordance with the invention, the photo diode region includes a p-type photo diode region formed on the surface of the photo diode region of the semiconductor substrate, a first n-type photo diode region underneath the p-type photo diode region, contacting an interface of the p-type photo diode region, and a second n-type photo diode region surrounding the first n-type photo diode region. It is preferable that impurities of the first n-type photo diode region have smaller projection distance and diffusivity than impurities of the second n-type photo diode region.
In accordance with another aspect, the invention is directed to a method for manufacturing an image sensor having a photo diode. In accordance with the method, a transfer gate and a reset gate are formed on predetermined portions of a semiconductor substrate with a specific distance therebetween. A photoresist pattern is formed to expose a region at one side of the transfer gate. A p-type photo diode region is formed by implanting p-type impurities into the exposed semiconductor substrate with a first ion implantation energy. A first n-type photo diode region is formed by implanting first n-type impurities into a portion below the p-type photo diode region, with a second ion implantation energy. A second n-type photo diode region is formed by implanting second n-type impurities into a portion surrounding the first n-type photo diode region, with a third ion implantation energy. The photoresist pattern is removed.
In one embodiment, the p-type impurities are boron ions. The first n-type impurities may be arsenic ions. The arsenic ions can be implanted with a concentration of 1xc3x971012/cm2 to 9xc3x971012/cm2, and the second ion implantation energy is greater than the first ion implantation energy.
The second n-type impurities may be phosphorus ions. The phosphorus ions can be implanted with a concentration of 5xc3x971011/cm2 to 9xc3x971011/cm2, and the third ion implantation energy is greater than each of the first and second ion implantation energies.
The steps of forming the p-type photo diode region, forming the first n-type photo diode region, and forming the second n-type photo diode region can be performed in different temporal orders.
In addition, after removing the photoresist pattern, spacers can be formed on both walls of the transfer gate and the reset gate, and a junction region can be formed by implanting the n-type impurities into a space between the transfer gate and reset gate, and to one side of the reset gate.